1. Field of the Invention
The present invention relates to a method of drying a semiconductor wafer, and more particularly, to a method for removing a residual solution, which comprises dissolved oxygen, from the semiconductor wafer.
2. Description of the Prior Art
During semiconductor fabrication processes, some metal ion, particle and organic compound may remain on the surface of the wafer. Native oxides may also form on the semiconductor wafer after cleaning, deposition, etching and conveying processes, degrading the quality of the semiconductor product. One or more cleaning processes are consequently employed to ensure the surface cleanliness of the semiconductor wafer.
In general, there are two types of cleaning processes: wet and dry. The wet cleaning process is wildly employed. After a wet cleaning process, a drying process is performed on the semiconductor wafer to remove any residual water as quickly as possible. The drying process is used to prevent water mark, and their associated defects, from forming on the wafer. Water mark occur when dissolved oxygen (DO) in the water reacts with the bare silicon on the surface of the semiconductor wafer.
Methods for drying semiconductor wafers according to the prior art include spin drying, IPA vapor drying and Marangoni drying, etc. Each of these drying methods has shortcomings. In order to improve the yield of the semiconductor manufacturing process, and also to avoid environmental pollution from chemical solutions used in some drying processes, a fast and effective drying process must be developed that does not use chemical solutions.
Please refer to FIG. 1. FIG. 1 is a cross-sectional diagram of spin drying a semiconductor wafer 10 according to the prior art. The surface of the semiconductor wafer 10 comprises many metal-oxide-semiconductor (MOS) transistors 14, trenches 16 and contact holes 18. Droplets 11 remain in the trenches 16 and contact holes 18 of the semiconductor wafer 10. Spin drying uses centrifugal force to remove the droplets 11 from the surface of the semiconductor wafer 10. The wafer 10 is spun at high speeds and at room temperature to quickly dry its surface. About six minutes per batch of wafers are required for the drying process. In FIG. 1, the arrow 17 indicates the direction of rotation of the semiconductor wafer 10 spinning around an axis of rotation 15. The arrows 19 indicate the centrifugal force during the spinning process.
In general, the wafer rotational speed must be at least 3500 rpm (revolutions per minute) to ensure a complete removal of the droplets 11 from the surface of the semiconductor wafer 10. However, excessive rotational speeds can lead to damage to the electric devices on the semiconductor wafer 10. The speed of rotation is therefore usually set to 3000 rpm when spin drying according to prior art. Because the rotational speed is insufficient, water marks form on the semiconductor wafer 10. Furthermore, dissolved oxygen in the water marks can cause additional defects on the semiconductor wafer 10.
When using the prior art spin drying method to dry the semiconductor wafer 10, the centrifugal drying effect is reduced due to the surface structure of the semiconductor wafer 10. This structure comprises many trenches and walls, which are a consequence of the many devices on the surface. The droplets 11 are trapped within the trenches 16 and contact holes 18, and so are difficult to remove by the centrifugal force 19 of the spin drying method. Another problem with spin drying is the issue of static charge. During the high-speed spinning of the semiconductor wafer 10, it accumulates static charge. This charge attracts particles in the air, thereby reducing the surface cleanliness of the semiconductor wafer 10.
Please refer to FIG. 2. FIG. 2 is a schematic diagram of a prior art IPA vapor drying process of the semiconductor wafer 10. In FIG. 2, the arrows 21 indicate the vertical direction of an upwardly flowing IPA (isopropyl alcohol) vapor. This IPA vapor drying process uses a heater 32 to evaporate an IPA solution 12, thereby forming the IPA vapor. Next, the semiconductor wafer 10 is placed inside the heated vapor of the IPA solution 12, and condensed IPA replaces the water adhering to the surface. Finally, the water and the condensed IPA are together taken away through a collector 36 and a pipe 38, completing the drying of the semiconductor wafer 10. About 10 min/batch is required for the IPA vapor drying process.
However, the droplets 11 in the trenches 16 and contact holes 18 are not easily replaced by condensed IPA 12. The IPA process does, however, avoid charge accumulation. It also requires the use of a great deal of IPA solution, which can lead to a lot of environmental pollution.
Please refer to FIG. 3. FIG. 3 is a side view of a prior art Marangoni drying process of a semiconductor wafer 20. The Marangoni drying method involves slowly removing the semiconductor wafer 20 vertically from a washing tank (not shown), at a suitable speed and at room temperature, then using a nitride gas 23 and an IPA vapor 21 to blow dry the semiconductor wafer 20. When the semiconductor wafer 20 is exiting from the surface 24, a bent region is formed between the semiconductor wafer 20 and a meniscus 22 between gas and liquid, into which the IPA vapor 21 dissolves. This reduces the surface tension of the water, preventing the semiconductor wafer 20 from dripping water. The advantage of the Marangoni drying method is that it uses less IPA solution. But it is still difficult to replace the droplets of water in trenches and contact holes with condensed IPA.